More on the OA impact advantage

Philip M. Davis, Citation advantage of Open Access articles likely explained by quality differential and media effects, PLoS Biology, a letter to the editor, January 16, 2007.  Here's the abstract from Davis' self-archived edition of the letter: 

In a study of articles published in the Proceedings of the National Academy of Sciences, Gunther Eysenbach discovered a significant citation advantage for those articles made freely-available upon publication (Eysenbach 2006). While the author attempted to control for confounding factors that may have explained the citation differential, the study was unable to control for characteristics of the article that may have led some authors to pay the additional page charges ($1,000) for immediate OA status. OA articles published in PNAS were more than twice as likely to be featured on the front cover of the journal (3.3% vs. 1.4%), nearly twice as likely to be picked up by the media (15% vs. 8%) and when cited reached, on average, nearly twice as many news outlets as subscription-based articles (4.2 vs. 2.6). The citation advantage of Open Access articles in PNAS may likely be explained by a quality differential and the amplification of media effects.